1. Field of the Invention
The present invention relates to an electrically conductive roll such as a charging roll, a developing roll, or a transferring roll, for use in an electrophotographic copying machine, printer, etc.
2. Discussion of Related Art
Electrically conductive rolls such as a charging roll, a developing roll, and a transferring roll are used in an electrophotographic copying machine, printer, etc., so that those rolls perform respective functions.
For instance, the charging roll is used in a roll charging method wherein a photosensitive drum on which an electrostatic latent image is formed is charged by the charging roll. Described more specifically, in the roll charging method, the photosensitive drum and the charging roll are rotated such that the charging roll to which a voltage is applied is held in pressing contact with an outer circumferential surface of the photosensitive drum, to thereby charge the outer circumferential surface of the photosensitive drum. The developing roll carries a layer of toner on its outer circumferential surface. The photosensitive drum and the developing roll are rotated such that the developing roll is held in contact with the outer circumferential surface of the photosensitive drum on which the electrostatic latent image is formed, so that the toner is transferred from the developing roll onto the photosensitive drum, whereby the latent image is developed into a toner image. The transferring roll transfers the toner image developed by the toner supplied from the developing roll, onto a recording medium such as a sheet of paper.
Such conductive rolls include a suitable shaft body (core metal) as an electrically conductive body and a conductive elastic layer formed on an outer circumferential surface of the shaft body and provided by a rubber layer having a relatively low hardness. The conductive rolls further include, as needed, a resistance adjusting layer and a protective layer formed in the order of description on an outer circumferential surface of the conductive elastic layer. The conductive rolls are needed to have high degrees of surface smoothness and dimensional accuracy for assuring that the conductive rolls are held in uniform contact with the photosensitive drum, etc.
The conductive rolls described above are conventionally produced, for example, (1) by using a cylindrical mold as disclosed in Patent Document 1 (JP-A-8-190263) or (2) by using an extruder as disclosed in Patent Document 2 (Japanese Patent No. 3320001). Described in detail, in the method (1), a shaft body is positioned in a mold cavity of the cylindrical mold such that the shaft body is positioned at the center of the mold cavity. An unvulcanized rubber composition for a conductive elastic layer is introduced into the mold cavity such that an annular space of the mold cavity around the shaft body is filled with the unvulcanized rubber composition. Thereafter, the unvulcanized rubber composition is vulcanized, so that the elastic layer is formed integrally on the outer circumferential surface of the shaft body. As needed, a resistance adjusting layer, a protective layer, etc., are formed in this order on the outer circumferential surface of the elastic layer. In the method (2), a tubular body (cylindrical body) formed of an unvulcanized rubber composition for a conductive elastic layer is fabricated by using the extruder. A shaft body is disposed within an inner bore of the tubular body. The tubular body formed of the unvulcanized rubber composition is vulcanized, so that the elastic layer is formed integrally on an outer circumferential surface of the shaft body. As needed, a resistance adjusting layer, a protective layer, etc., are formed in this order on the outer circumferential surface of the elastic layer.
The conductive roll produced according to the above-described method (1) has high degrees of surface smoothness and dimensional accuracy. The method (1), however, needs a mold having a mold cavity which has a configuration corresponding to that of the integral structure consisting of the shaft body and the elastic layer, undesirably suffering from low efficiency and high cost of manufacture of the conductive roll.
The above-described method (2) effectively reduces the required time and cost of manufacture of the conductive roll. Where the elastic layer is formed according to the method (2) by using a rubber composition having a relatively large die swell value which represents a ratio of expansion of the rubber upon extrusion from a die of the extruder, the rubber composition is not extruded with high efficiency. In this case, the surface of the extruded tubular body that gives the elastic layer is not sufficiently smoothed, in other words, the surface is undesirably rough with concavities and convexities. In addition, the extruded tubular body, and accordingly the elastic layer does not have a high degree of dimensional accuracy. Described more specifically, if a conductive roll whose outer surface is rough is used as the charging roll, the toner adheres to the outer surface of the roll, making it impossible to uniformly charge the outer circumferential surface of the photosensitive drum. In this case, an image reproduced on a sheet of paper by using such a charging roll has a poor quality, that is, the entirety of the reproduced image is faded, or lines appear as a part of the reproduced image. If a conductive roll which does not have a high degree of dimensional accuracy is used as the charging roll, the roll is not held in uniformly pressed contact with the photosensitive drum, so that the outer circumferential surface of the photosensitive drum is not uniformly charged. In this case, lines undesirably appear as a part of the reproduced image in a transverse direction of the sheet of paper. Accordingly, where the conductive elastic layer of the roll is formed according to the above-described method (2), i.e., by extrusion, the outer surface of the roll needs to be ground or polished for increasing the degrees of surface smoothness and dimensional accuracy.
The conductive roll whose conductive elastic layer has a high degree of dimensional accuracy may be formed with reduced time and cost of manufacture if a resin composition having a relatively small die swell value is used for extrusion, in place of the above-described rubber composition having a relatively large die swell value. The elastic layer formed of the resin, however, is inferior in terms of a resistance to permanent set to the elastic layer formed of the rubber. Accordingly, the conductive roll having such an elastic layer formed of the resin is not held in uniformly pressed contact with the photosensitive drum with high stability. Accordingly, the outer circumferential surface of the photosensitive drum cannot be uniformly charged, so that a reproduced image may undesirably have a poor quality.